Method of coloring aluminum



Patented July 3, 1934 PATENT OFFICE v UNITED STATES 1,965,269 METHOD OF COLORING ALUMINUM Martin Tosternd, Arnold, Pa.,

assignor to Aluminum Company of America, Pittsburgh,

a corporation of Pennsylvania No Drawing. Application. May

Serial No. 534.443

8 Claims. (01. 148-6) This invention relates to the treatment out aluminum and aluminum alloy surfaces to produce thereon colored coatings, particularly colored coatings possessing good resistance to corrosion. The term aluminum as used herein and in the appended claims includes this metal in every degree of purity. The term aluminum alloys as used herein and in the appended claims includes all aluminum alloys having an aluminum content of 50 per cent or more.

The many and varied uses of aluminum in both the practical and decorative arts have made very desirable the development of a suitable process by which aluminum surfaces may be colored. The

production of brown and brownish black colors,

which colors are often desirable on aluminum surfaces, presents particular difliculties. Such colors can, however, be readily obtained by my present invention and the colors produced have the further advantage of being present in the form of a corrosion-resistant coating on the aluminum or aluminum alloy surface.

My invention comprises first treating the aluminum article to provide on its surface a hard and substantially adsorbent oxide coating and thereafter treating this coating to deposit therein an insoluble oxide of one or more metals by chemical means.

The aluminum article is first treated to provide upon its surface an oxide coating which is hard and dense and which has sufficient adsorbent qualities to retain, when immersed in or equivalently treated with a liquid, an appreciable quantity of such liquid within and upon it. The term oxide coating as used herein is a well known designation in the art to describe a layer of aluminum oxide artificially produced on the aluminum or aluminum alloy surfaces by chemical treatment, with or without the use of applied electrical energy, but the term does not include the thin film of aluminum oxide which is naturally formed upon the metal by contact with the air. After the oxide-coated aluminum or aluminum alloy has been prepared, I employ the novel methods of my invention to deposit in the oxide coating an insoluble metallic oxide.

The oxide-coated aluminum or aluminum alloy is placed in contact with a solution, preferably but not necessarily aqueous, of a soluble metallic .compound, the metallic ion of which will, when combined with oxygen, produce a metallic oxide of characteristic brown to brownish black color and/or of corrosion-inhibitive properties. I have discovered that when'the oxide-coated metal isplaced incontact with or immersed in a solution of a metallic salt, the solution may be adsorbed in and on the oxide coating and that when the metallic ion so adsorbedby the oxide coating is treated with a solution of an oxidizing agent, the metallic ion may be oxidized to form in the oxide coating .0

an insoluble metallic oxide. I have further discovered that the insoluble metallic oxide formed is so firmly and adherently deposited in the oxide coating as to become, in effect, an integral part of the coating and to impart to it certain corrosion- C5 resistant properties. 4

The solutions which I prefer to use in the preparation of an oxide-coated aluminum or aluminum alloy article are of three types,-sulfuric acid solutions, chromic acid solutions, or '0 sodium carbonate and potassium dichromate solutions. In forming an oxide coating on aluminum or its alloys by the use of solutions of sulfuric acid, aqueous solutions are preferably used, the concentration of the acid ranging from '16 about 1 to per cent. The metallic article to be coated is placed in the bath. and made the anode of an electric circuit. When a solution of '7 per cent of sulfuric acid is used, a currentof from about 0.01 to 0.4 amperes per square inch is impressed upon the article and the action of the acid solution on the aluminum or aluminum alloy is allowed to continue for about 30 minutes. When lower concentrations of acid are used, heating of the solution is preferred to increase the rapidity of the action. With higher concentrations of acid, lower current densities are preferably impressed on the anode. When chromic acid is used as the oxide coating solution, the conditionsnamed in the case of sulfuric acid may be followed substantially. In my preferred practice I have found a 3 per cent solution of chromic acid to give excellent results. When a solution of sodium carbonate and potassium dichromate is used to produce an oxide coating on aluminum or aluminum alloys I have found it preferable to simply immerse the piece to be coated in the solution without applying any external electric energy. A boiling solution containing about 0.5 to 6 per cent of sodium carbonate and about 0.1 to 1 per cent of potassium dichromate will give good results, but as a specific solution I prefer'one containing about 2 per cent of sodium carbonate and 0.5 per cent of potassium dichromate.

After the .oxide coating has been satisfactorily formed on the aluminum or aluminum alloy surface, I treat the coated metal to deposit therein an insoluble metallic oxide. The term "metallic oxide" as used herein and in the appended claims includes, in addition to the oxides of the metals,

' insoluble metallic oxides as used herein and in the appended claims refers to such metallic oxides as are substantially insoluble in water.-

The method of introducing these metallic oxides into the preformed oxide coating consists of two steps. The first step consists in placing the oxide-coated metal surface in contact with a solution containing the metallic ion of the desired metallic oxide in the form of a soluble compound. The oxide-coated metal is maintained in contact with the solution for a sumcient time to allow the solution to be adsorbed in or on the oxide coating. The second step consists in bringing the oxide-coated metal impregnated with the soluble metallic salt in contact with a second solution which is capable of oxidizing the soluble metallic salt to produce the insoluble metallic oxide or hydratedoxide desired. The insoluble metallic oxide is thus deposited in and on the oxide coating of the aluminum metal, producing therein a substantially permanent color.

As a specific example of the production of these metallic oxides of characteristic color in the oxide-coated aluminum, the oxide-coated metal may be first immersed in a solution of a soluble salt of lead, such as acetate or nitrate, and thereafter treated with a solution of any oxidizing agent, such for example, as potassium permanganate or a solution of a hypochlorite or chlorine water or any other suitable oxidizing agent. The solution of lead acetate may be of any desired concentration but I prefer to use solutions containing up to about 20 per .cent of lead acetate. As an' oxidizing agent I prefer to use a 5 to 10 per cent solution of potassium permanganate at a temperature of about 70 C. 'With this combination of reagents I obtain a brown colored coating which is substantially corrosion-resistant. The

shades of coating may be varied by varying the amount of oxide of lead deposited therein, that is, by varying the concentration of the solution of soluble lead salt with which the oxide coating is first treated. The use of permanganate also affacts the shade of coating produced, perhaps due to the fact that an oxide of manganese is deposited along with the insoluble lead oxide.

For very dark brown to black colors I may use 'cobalt oxides as the coloring agents. In producing these colors I treat the oxide-coated aluminum with a solution containing up to about 20 per cent of soluble cobalt salt, for example, acetate,

nitrate, sulphate or chloride, and subsequently :treat the impregnated oxide coating with a solution of an oxidizing agent at a temperature of about 70 C. Again, I prefer to use a 5 to 10 per cent solution of potassium permanganate as my oxidizing agent, though solutions of other oxidizing agents may be used.

In practicing the steps of my novel process I have determined that the concentration of the solutions in which the oxide-coated metal is immersed is not a critical factor and that solutions containing as little as 1 per cent of the soluble compounds used, or solutions which are completely saturated with the soluble compounds, will produce the colors desired. However, I prefer to use the more concentrated solutions within the range described in the above examples since I have observed that in more concentrated solution the adsorption of the solution by the oxide coating is more rapid than in dilute solu tion. I have further determined that the temperature of the solutions at the time at which the oxide-coated metal is introduced therein is not a governing factor and that satisfactory colors may be produced whether the solutions be hot or cold. As pointed out in the above examples, however, I prefer to use a temperature of about 70 as I thereby obtain a more rapid and complete oxidation of the metal.

It is to be understood that the invention is not limitedvto the specific detailsherein described since these can be varied without departure from the spirit of the invention.

I claim- 1. The method of coloring the surface of an aluminum or aluminum-alloy article, comprising forming on the metal surface an adsorbent oxide coating and treating such surface with successlve solutions, one containing in solution a metallic compound'capable of being converted into oxide ing said compound to an oxide, whereby saidcompound and oxidizing agent are adsorbed by the coating and react to precipitate therein a metallic oxide of characteristic color.

2. The method of coloring the surface of an aluminum or aluminum-alloy article, comprising forming on the metal surface an adsorbent oxide coating and treating such suri'ace with successive solutions, one containing a lead compound capable of being converted into oxide by an oxidizing agent in solution, and the other containing an oxidizing agent capable of converting said compound to an oxide, whereby said compound and oxidizing agent are adsorbed by the coating and react to precipitate therein a brown metallic oxide.

3. The method of coloring the surface of an aluminum or aluminum-alloy article, comprising forming on the metal surface an adsorbent oxicle coating and treating such surface with successive solutions, one containing a lead compound capable of being converted into oxide by an oxidizing agent in solution, and the other containa coating and treating such surface with successive solutions, one containing in solution a cobalt compound capable of being converted into oxide by an oxidizing agent in solution, and the other containing an oxidizingagent capable of converting said compound to an oxide, whereby said compound and oxidizing agent are adsorbed by' the: coating and react to precipitate therein a'darlr brown metallic oxide.

6. The method of coloring the surface of an aluminum or aluminum-alloy article comprising forming on the metal surface an adsorbent oxide coating and treating such surface with successive solutions, one containing in solution a cobalt compound capable of being converted into oxide by an oxidizing agent in solution, and the other containing potassium permanganate, whereby said compound permanganate are adsorbed by the coating and react to precipitate therein a dark brown metallic oxide.

7. The method of treating the surfaces of an aluminum or aluminum alloy article comprising forming on the metal surface an adsorbent oxide coating and treating such surface with a solution of cobalt acetate and a solution of potassium permanganate to precipitate a dark brown oxide in said coating.

.' 8. The method of coloring the surface of an aluminum or an aluminum-alloy article, comprising forming on the metal surface an adsorbent oxide coating and treating such surface with successive solutions, one containing in solution a metallic compound capable of being. converted into oxide by an oxidizing agent in solution, and the other containing suchoxidizing agent of the class composed of permanganate, hyp'ochlorite, and chlorine, whereby said compound and oxidizing agent are adsorbed by the coating and react to precipitate therein a metallic oxide of characteristic color.

MARTIN TOSTERUD.

CERTIFICATE 01+" CORRECTION.

Pa en Na 1, 965; 269,

,lllly 3, 1934,

MARTIN TOSTERUD.

It is hereby certified that error appears in the printed specifica ion of the above numbered patent requiring correction as follows: Page 2, line [50, claim 6, after "compound" insert the word and;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 7th day. of August, A. D. 1934.

7 (Seal) Leslie Frazer Acting Commissioner of Patents.

by the coating and react to precipitate therein a dark brown metallic oxide.

7. The method of treating the surfaces of an aluminum or aluminum alloy article comprising forming on the metal surface an adsorbent oxide coating and treating such surface with a solution of cobalt acetate and a solution of potassium permanganate to precipitate a dark brown oxide in said coating.

.' 8. The method of coloring the surface of an aluminum or an aluminum-alloy article, comprising forming on the metal surface an adsorbent oxide coating and treating such surface with successive solutions, one containing in solution a metallic compound capable of being. converted into oxide by an oxidizing agent in solution, and the other containing suchoxidizing agent of the class composed of permanganate, hyp'ochlorite, and chlorine, whereby said compound and oxidizing agent are adsorbed by the coating and react to precipitate therein a metallic oxide of characteristic color.

MARTIN TOSTERUD.

CERTIFICATE 01+" CORRECTION.

Pa en Na 1, 965; 269,

,lllly 3, 1934,

MARTIN TOSTERUD.

It is hereby certified that error appears in the printed specifica ion of the above numbered patent requiring correction as follows: Page 2, line [50, claim 6, after "compound" insert the word and;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 7th day. of August, A. D. 1934.

7 (Seal) Leslie Frazer Acting Commissioner of Patents. 

